1. Technical Field
Methods and example implementations described herein are generally directed to disk drives, and more specifically, to throttling host commands to disk drives.
2. Related Art
New schemes are being evaluated for using the cache memory of a disk drive or a solid state disk (SSD) portion of a device, to mitigate vibration and other issues seen by the rotating media of the disk drive, or a hard disk drive (HDD) portion of the device.
One of the schemes is to direct all new host write commands to the cache when the disk drive or device is subjected to different environmental conditions. For example, under persistent vibration conditions, draining of data from the cache/SSD portion to rotating media/HDD portion is much slower resulting in saturation (i.e., filling) of the cache. Once the cache saturates, performance will drop dramatically as the disk drive or device is thereby limited to the write capability of the HDD portion/rotating media under vibration, which is often very limited. Such conditions may induce inconsistent swings in performance over time.
FIGS. 1(a) and 1(b) illustrate example related art operation flows during a non-vibration mode operation and a vibration mode operation, respectively. As illustrated in FIG. 1(a), write host commands 200 are transmitted to both the cache 101 and the rotating media 103 of a disk drive when the disk drive is not undergoing adverse vibration conditions. As illustrated in FIG. 1(b), when the disk drive undergoes adverse vibration conditions, the write host commands 200 are sent only to the cache 101, which is drained to the rotating media 103. Because the rate of writing to the cache can greatly exceed the rate of writing to the rotating media (which is typically very low during vibrations), prolonged operation of sending the write commands only to the cache 101 will eventually result in cache saturation. The same situation can also take place in a device with an HDD portion and an SSD portion used as a cache to the HDD portion.
FIG. 2 illustrates an example vibration throughput plot for the example operation flow of FIG. 1(b). As illustrated in FIG. 2, the throughput during vibration mode operation may remain adequate, as shown at 300, until the cache becomes saturated, whereupon the throughput performance may decrease drastically, as shown at 301. The likelihood of the cache becoming saturated may increase significantly when the device or disk drive operates under vibration mode over an extended period of time. Cache saturation may eventually lead to the disk drive or device not being able to timely process data access commands, which in turn may cause an operating system of the host to hang or go into an unrecoverable error state.